1. Field of the Invention
This invention relates to solid state relays and, more particularly, to methods and apparatus for reducing the level of electro-magnetic interference emissions (EMI) causes by commutation voltage spikes.
2. History of the Prior Art
One type of prior art solid state AC relay uses silicon controlled rectifiers (SCRs) as switching elements to provide current to an output load device from a source of alternating current. An SCR is a type of semiconductor device that is triggered into conduction by injecting a control current into its gate terminal. Once triggered, the SCR will remain in the on condition and conduct current with low voltage drop until there is no longer a sufficient anode current available or until the anode current drops below the minimum latch current of the SCR. This characteristic of an SCR by which it turns off at near zero current makes it a valuable switching element for solid state AC relays because switching at low current generates much less electro magnetic interference (EMI) than is generated by a typical switch.
By utilizing a pair of such SCR devices in inverse parallel arrangement connected across the load and the source of current, the current to the load may be controlled. It has become typical to utilize an input circuit for the output SCR switching elements which is itself a relay constructed of a pair of light sensitive SCR switching elements and features both the zero current turn-off characteristic mentioned above and a zero voltage turn-on characteristic. A zero voltage turn-on circuit is so called because the relay tends to switch on in a range at which the voltage being controlled has a minimal value. For example, one such circuit might switch in a range of plus or minus ten volts for an AC source voltage varies from plus 250 volts to minus 250 volts RMS. Both zero voltage turn-on and zero current turn-off features have been found desirable because they tend to eliminate a great deal of the transient problems which occur if the relay is switched on at an instant when the AC voltage or current being controlled has a substantial value.
The input circuit used by the prior art is optically-coupled to a source of control signals which may be provided by digital control circuitry. Such optical coupling provides excellent electrical isolation between the logic signals and the AC loads which they control. An input signal applied through the optically-coupled relay input portion will turn on one of the output SCRs allowing it to conduct and furnish current to the load until the current to the anode of the output SCR fails below an amount sufficient to maintain conductance. At this point, the AC source current changes polarity and switches on the other of the output SCRs which responds in the same manner to apply current to the load in the opposite sense.
However, it has been discovered that in cases involving high load currents, these prior art zero voltage turn-on and zero current turn-off circuits will generate an amount of EMI noise which is unacceptable for many applications.